Using ExecuteURL as 404 handler in web.config will bypass URL Rewrite (ie.. outboundRules) while using other responseModes won't

Question

I have the following rule in web.config designed to identify and rewrite outbound session cookies with both the secure and httpOnly flags:

<rewrite>
    <outboundRules>
        <preConditions>
            <preCondition name="MatchSessionCookies">
                <add input="{RESPONSE_SET_COOKIE}" pattern="." />
            </preCondition>
        </preConditions>

        <rule preCondition="MatchSessionCookies" name="SecureSessionCookies" enabled="true">
            <match serverVariable="RESPONSE_SET_COOKIE" pattern="^(.*sess.*)=(.+)$" />
            <action type="Rewrite" value="{R:1}={R:2}; httpOnly; secure" />
        </rule>
    </outboundRules>
</rewrite>

This works as intended, up until httpErrors comes into play:

<httpErrors>
    <remove statusCode="404" subStatusCode="-1" />
    <error statusCode="404" prefixLanguageFilePath="" path="/path/to/404.aspx" responseMode="ExecuteURL" />
</httpErrors>

So when accessing /a-page-that-exists.aspx, the outbound ASPSESSIONID cookies that get written out are successfully rewritten with both secure and httpOnly flags.

Request URL: /a-page-that-exists.aspx
Status Code: 200 OK

Set-Cookie: ASPSESSIONIDABCDEFG=...; path=/; httpOnly; secure

The problem is accessing /a-page-that-does-NOT-exist.aspx. It appears that the [404] request is internally "routed" to the ExecuteURL path and my URL rewrite rules I have in place are bypassed altogether.

Request URL: /a-page-that-does-NOT-exist.aspx
Status Code: 200 OK

Set-Cookie: ASPSESSIONIDABCDEFG=...; path=/

Any ideas on how to modify my outbound rewrite rules so that they can be applied to [404] requests before being handed of to my 404 handler?

Answer 1

Well, it looks like we have to make do with a URL Rewrite version of IIS <httpErrors /> handler, but it works:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <!-- Remove existing 404 handler -->
        <httpErrors>
            <remove statusCode="404" subStatusCode="-1" />
        </httpErrors>

        <rewrite>
            <outboundRules>
                <preConditions>
                    <preCondition name="MatchSessionCookies">
                        <add input="{RESPONSE_SET_COOKIE}" pattern="." />
                    </preCondition>
                </preConditions>

                <!-- Does NOT work with ExecuteURL 404 handler -->
                <rule preCondition="MatchSessionCookies" name="SecureSessionCookies" enabled="true">
                    <match serverVariable="RESPONSE_SET_COOKIE" pattern="^(gsm|.*sess.*)=(.+)$" />
                    <action type="Rewrite" value="{R:1}={R:2}; httpOnly; secure" />
                </rule>
            </outboundRules>
            <rules>
                <!-- Re-implement ExecuteURL 404 handler as URL Rewrite -->
                <rule name="Handle404" stopProcessing="true">
                    <match url=".*" />
                    <conditions logicalGrouping="MatchAny">
                        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                    </conditions>
                    <action type="Rewrite" url="/path/to/404.aspx?404;{PreserveSchema:{HTTPS}}{HTTP_HOST}{UNENCODED_URL}" />
                </rule>
            </rules>
            <rewriteMaps>
                <!-- http://stackoverflow.com/a/10227936/901156 -->
                <rewriteMap name="PreserveSchema" defaultValue="OFF">
                    <add key="ON" value="https://" />
                    <add key="OFF" value="http://" />
                </rewriteMap>
            </rewriteMaps>
        </rewrite>
    </system.webServer>
</configuration>

And the response:

Request URL: /a-page-that-does-NOT-exist.aspx
Status Code: 200 OK

Set-Cookie: ASPSESSIONIDABCDEFG=...; path=/; httpOnly; secure